Precipitating sand and gravel anti-pollution dryer for asphalt plants

ABSTRACT

A drying apparatus having a chamber for condensing hot gases and steam on incoming sand and gravel aggregate. The resultant warm wet aggregate washes dust from the exhausting air of the dryer. A great reduction in fuel consumption and air pollution by reclaiming heat energy from the steam given off when drying aggregates.

United States Patent Darby 51 July 25, 1972 [54] PRECIPITATING SAND AND GRAVEL ANTI-POLLUTION DRYER FOR ASPHALT PLANTS [72] Inventor: Kenneth S. Darby, RD. ll, Falls, Pa.

[22] Filed: Jan. 20, I971 [2l] Appl.No.: 108,057

[52] [1.5. CI .34/167, 34/168, 34/l 77 [5|] lnLCI ..F26b l7/l2 [58] Field of Search ..34/33, 137, I67, I68, I70,

[56] References Cited UNITED STATES PATENTS l,308,942 7/l9l9 French ..34/l67X 3,251,140 5/1966 Fraenzel ..34/l37 FOREIGN PATENTS OR APPLICATIONS l, l07,599 5/l 96l Germany ..34/166 Primary Examiner-Carroll B. Dority, .Ir.

l 5 ABSTRACT A drying apparatus having a chamber for condensing hot gases and steam on incoming sand and gravel aggregate. The resultant warm wet aggregate washes dust from the exhausting air of the dryer. A great reduction in fuel consumption and air pollution by reclaiming heat energy from the steam given off when drying aggregates.

1 Claim, 3 Drawing Figures Patented July 25, 1972 INVENTOR FIG. 1

PRECIPITAIING SAND AND GRAVEL ANTI-POLLUTION DRYER FOR ASPHALT PLANTS This invention relates generally to a method of drying and heating sand and gravel for asphalt mixing plants. It is a more economical method than any prior methods of drying and heating sand and gravel. More specifically, it greatly reduces the amount of pollutant exhaust produced by present methods of heating and drying sand and gravel for asphalt mixing plants.

The production of asphalt for road paving requires drying sand and gravel and heating it to over 300 F. before mixing it with hot, liquid asphalt. Present methods of drying the sand and stone aggregate utilize a large turning drum into which at one end, air and fuel are forced and ignited. The sand and gravel mixture enters through the other end and tumbles through the open flame creating a great amount of dust during the drying process.

The exhaust from this heating chamber contains the fuel residue, hot gases, steam, and dust from the aggregate. To eliminate polluting air it is necessary to have a dust collector system, followed by an air scrubbing or washing system. This air cleaning system would cause such a back pressure on the dryer that the burner would not operate without a large fan to pull the hot gases from the burner. These fans usually require over I horsepower to do this job. In this process the air and hot gases are pulled with such a rapid velocity that much of the heat value of the steam and the hot gases goes off into the atmosphere without any value to drying and heating the sand and gravel.

The washing of the air requires water from an outside source and large settling basins for the contaminated water.

Because of the inefficiency of these old methods of drying aggregates, a great amount of fuel is burned and a high velocity of air is needed, exposing all the equipment to extremely high temperature and abrasive erosion. These high temperatures and the erosion deteriorate the equipment and it must be replaced or rebuilt at regular intervals.

ADVANTAGES OF THIS INVENTION There are numerous advantages in drying sand and gravel in a more efficient manner, the two greatest being to nearly eliminate air pollution and to lower the consumption of fuel required to do the job.

This present invention of precipitating sand a gravel in a chamber absorbs the heat from the steam, settles the dust, washes the air without the need of an outside water source, and eliminates air scrubbers, large settling basins, extensive duct work, and dust accumulators.

About 50 percent of the energy of the fuel burned in a conventional dryer leaves the dryer in the form of steam. This steam, because of its great expansion, increases the velocity of exhaust gases and is a great waste of heat energy.

This present invention keeps the velocity of the hot gases and air as low as possible to eliminate carrying dust and to allow recapturing of the steam. This method condenses the steam in the chamber, reducing the volume of the steam and reclaiming the heat of vaporization or 966.6 BTU/lb. steam.

One pound of steam at 2l2 F. occupies L700 times more space than I lb. of water at 2| 2 F. In an asphalt dryer as much as 20,000 lb. of water per hour is removed from the sand and gravel in the form of steam occupying over one-half million cubic feet of space and containing usable latent heat. This costly steam is exhausted to the atmosphere as waste. In this present invention we remove the heat of vaporization from the steam by condensing it on the cool aggregates falling in the first and second chambers, and then draining the water from the bottom of the chambers. This process reduces the volume of hot gases in the form of steam to a liquid occupying L700 times less space, thus greatly reducing the velocity of hot gases. This would reduce the velocity of the escaping gases by over 300 feet per minute in a stack with a cross sectional area of 30 square feet.

A high velocity of gases passing through dry sand and gravel carry a lot of dust. Precipitating aggregate dryer reduces the velocity of exhausting gases to a minimum and the cool falling damp aggregate washes the warm exhaust air.

This invention is unique in the manner in which the aggregate is heated. Instead of an open flame and tumbling the aggregate in one chamber, this invention heats the aggregates in three different stages. The fuel is burned in two or three large scotch marine type boilers. These units work at atmospheric pressure and heat the heat transferring oil to over 400 F. This hot oil is pumped through the final stage of the dryer, a heat exchanger, consisting of randomly spaced hot pipes which heat and baffle the fall of the aggregates as they drop in the final heat chamber. These hot pipes radiate heat in the final chamber and also give off heat by conduction to the aggregate which rests on top of them. The only movement of gas or air in the final chamber is that of steam given off from the hot aggregates. This steam leaves the top of the final stage and enters the top of the middle chamber and moves in a downward direction as it condenses on the aggregates rising in that middle chamber, thus heating them almost to boiling point. This center chamber also receives the exhausted or chimney gases from the scotch marine boilers. The sand for the asphalt enters the top of the middle chamber and falls into the downward stream of hot gases in that chamber. A certain percentage of the gases from the fuel burned produces water in the form of steam and this condenses on the falling aggregate, reducing the volume and velocity of exhaust gases. This falling sand cools and contracts the other gases in the downward exhaust stream.

The center and first chamber are connected by a large opening at the bottom of the chambers. This opening is large enough to allow an accumulation of aggregate with enough space above it for the natural draft of the first chamber or chimney to keep the furnaces operating efficiently. From this area on bottom, water is drained and sent to a small settling pond.

The first chamber or chimney has an opening near the top where the heavier aggregate or gravel is fed in. There are baffles in this chimney which slow the fall of the gravel and spread it over the entire area of the chimney.

This precipitating aggregate dryer would be approximately 60 feet in height and I0 feet square allowing it to be lowered and easily moved if necessary.

The upper section of the first chamber is a chimney having approximately twice the cross sectional area as the lower section. This in turn reduces the velocity of the warm exhaust gases, allowing dust particles to fall with the gravel which is wet from the condensing steam.

THE DRAWINGS A more detailed explanation of the invention is provided in the following description and claims, and is illustrated in the accompanying drawings, in which:

FIG. 1 is a cut open sectional view showing the three chambers inside the dryer.

FIG. 2 is a side view of the dryer showing the sand feed and gravel feed, the boilers, section B an opening into chamber 2, and section C an opening into chamber 1.

FIG. 3 is a top view of the dryer showing the location of the sand feed and gravel feed and the sectional view of 3 chambers through section DD.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIM ENT This precipitating aggregate dryer consists of three vertical chambers l, 2, and 3. The first and second being the condensing chambers, l and 2, where the steam and hot gases from the furnace are condensed on the precipitates which fall to the accumulating area 4 and are then raised in buckets 7 and dropped through opening 14 into chamber 3, the final heat exchanger. The hot dry aggregate accumulates at the bottom of the final chamber 3 and leaves through chute to the asphalt mixing equipment.

The fuel is burned in boilers l3 heating a hot transfer oil to about 400 F. and circulating it through the heating tubes 8 in final chamber 3. These boilers l3 operate at atmospheric pressure, are thermostatically controlled, and circulate the heating oil continuously while in operation. The boilers l3 exhaust the chimney fumes through pipes l5 downward into chamber 2. The sand feed ll carries the finer aggregate and drops them through a small opening 16 into chamber 2 above the incoming furnace exhaust pipe 15. Baffles l8 slow the fall of the precipitating sand as it drops in the stream of hot gases from boilers 13. This warms the sand and cools the exhaust gases as they condense on the sand. The sand accumulates at the bottom of chamber 2 to be carried upward by bucket conveyor 7.

The coarse aggregates are brought up gravel feed 12 and enter chamber 1 at small opening 17. The fall of the gravel in chamber l is retarded as it hits baffles 9 which break the fall. The gravel and sand that fell in chambers l and 2 accumulate at bottom area 4 to be raised by the buckets 7.

The hot gases enter from boiler chimneys 15 in a downward direction in chamber 2 and proceed into chamber 1 through opening 6. They rise in chamber 1 through the precipitating gravel and proceed out through the top of chamber 1. As they travel up chamber 1 the steam is condensed on the cool precipitating gravel which is slowed by baffles 9 and the gravel absorbs heat from the hot gases and steam as it falls to the bottom 4.

The aggregate absorbs heat again as it rises in the buckets from area 4 and is dumped into final chamber 3 over the heating pipes and baffles 8.

What I claim is:

1. A dryer for sand and gravel comprising, means forming three vertical drying chambers, a first of said chambers including vertically spaced horizontal baffles, a gas outlet at its upper end and a means for supplying sand and gravel into its upper end, said first chamber being open at its lower end to a second of said chambers, said second chamber being open at its upper end to a third of said drying chambers, means for conveying sand and gravel from the bottom of said second chamber and dumping it into said third chamber through said opening between said second and third chambers, a means of draining water out of bottom of said first chamber, heating tubes vertically spaced in said third chamber, a boiler, means for supplying flue gases from said boiler into the upper portion of said second chamber, means for supply a heating fluid from said boiler through said heating tubes in said third chamber, and means for withdrawing sand and gravel from the bottom of said third chamber.

I n l t 

1. A dryer for sand and gravel comprising, means forming three vertical drying chambers, a first of said chambers including vertically spaced horizontal baffles, a gas outlet at its upper end and a means for supplying sand and gravel into its upper end, said first chamber being open at its lower end to a second of said chambers, said second chamber being open at its upper end to a third of said drying chambers, means for conveying sand and gravel from the bottom of said second chamber and dumping it into said third chamber through said opening between said second and third chambers, a means of draining water out of bottom of said first chamber, heating tubes vertically spaced in said third chamber, a boiler, means for supplying flue gases from said boiler into the upper portion of said second chamber, means for supply a heating fluid from said boiler through said heating tubes in said third chamber, and means for withdrawing sand and gravel from the bottom of said third chamber. 